Affordable, “personal” jet aircraft are fast becoming a reality in the general aviation market, very small turbofan engines are required for power. Such aircraft require “very small” turbofan engines (i.e. 2000 pounds thrust and under) which can be economically operated by the general aviation pilot. Small scale turbofan gas turbine engines are known for use in expendable missiles in the delivery of military ordinance, however considerations such as cost-effective, affordable and efficient operation, and durability measured in thousands of hours (not minutes), have been irrelevant to their designers. Such prior art missile engine designs, therefore, provide none of the key deliverables required for such a market to be realized. Likewise, industrial microturbines are available, but their designs are ill-suited for use as an aircraft prime mover, for obvious considerations such as weight and size.
Scaling down of conventional civilian non-expendable turbofan engines, however, also presents difficulties due mainly to the disproportionate scaling of certain factors, such as strength to weight and tolerances. For example, non-expendable turbofan engines typically have a segmented case assembly, mainly for weight reduction reasons, but also to facilitate fabrication and assembly. A conventional case assembly 200 is illustrated in FIG. 1, and includes a fan case 202, an intermediate case 204, a compressor case 206, a gas generator case 208, a turbine case 210 and a turbine exhaust case 211 about centreline 212. The gas generator case 208, turbine case 210 and turbine exhaust case 211 surround the hot section of the engine and are typically made of steel or nickel alloys, which have good thermal resistance properties. However steel is relatively heavy, and therefore cooler portions such as the intermediate case 204 and the compressor case 206 typically employ lighter materials such as magnesium and/or aluminium. Steel is conventionally used for the fan case 202 because its strength is desirable for containing blade-off events.
A similar prior art configuration 300 is illustrated in FIG. 2, a case assembly 300 (only the upper half of which is shown), having a fan case 344, an intermediate case 346, and a gas generator case 352 (the turbine and exhaust cases are not shown) bolted together, along centreline 312. A compressor shroud 348 for encircling the compressor blades is bolted to the intermediate case 346, as is a bearing seat (not shown) at location 357. Flange connections 302, 304 and 306 are provided to accommodate differences in thermal expansion rates amongst the different material case components. Typically the case components are assembled in stages, as the engine component top-level assemblies are assembled therein.
Simply scaling down these larger case designs, however, becomes problematic in “very small” turbofan engines (i.e. generally 2000 pounds thrust, and less) for several reasons. One is the associated tolerance “stack-up”, which typically does not scale (i.e. the accuracy of manufacturing and assembly process does not increase as part size decreases). In typical turbofan engines, tolerance stack-up is less critical because it is small compared to the size of the components. But when considering blade tip clearance for example, the tolerance stack-up can have a very significant effect on the overall efficiency of a very small turbofan engine, since specific fuel consumption (SFC) is directly related to blade tip clearance. Any blade tip clearance must account for a tolerance stack-up, to avoid tip rubs caused by an unfavourable stack-up, and so tolerance stack-up directly affects efficiency. Another scaling problem is that factors often scale at different rates. For example, a component reduced to nominally half its original size, may not necessarily be halved in weight.
Another aspect which presents challenges to scaling down size is the differences in thermal expansion rates, which requires compensation and thereby adds weight and complexity. For example, the accessory gear box (AGB) tower shaft typically requires a telescoping design (and associated bearings) to account for thermal expansion differential. In the very small turbofan engine, such accommodations make the engine unfeasible expensive and inefficient to operate.
Therefore, as the affordable general aviation turbofan engine market develops, significant design problems are presented to the designer. Scaled-down turbofans are simply inefficient and heavy, and thus too expensive to operate in the general aviation market. Civilian version of expendable missile engines and airborne version of microturbines are also ineffectual solutions to the design problems presented. Thus, it is important to address the design problems of the very small turbofan engine.